1. Field of the Invention
The present invention relates to fiber optic gyroscopes. More particularly, this invention pertains to an improved sensor coil design that addresses factors contributing to various bias errors and minimizes gyro bias sensitivity to dynamic thermal and vibration environments.
2. Description of the Prior Art
A fiber optic gyroscope comprises the following main components: (1) a light source, (2) a beamsplitter (either a fiber optic directional coupler or an integrated-optics Y-junction), (3) a fiber optic coil, (4) a polarizer (and sometimes one or more depolarizers), and (5) a detector. Light from the light source is split by the beamsplitter into copropagating and counterpropagating waves travelling in the sensing coil. The associated electronics measures the phase relationship between the two interfering, counter-propagating beams of light that emerge from opposite ends of the coil. The difference between the phase shifts experienced by the two beams provides a measure of the rate of rotation of the platform to which the instrument is fixed.
Environmental factors can affect the measured phase shift difference between the counterpropagating beams, thereby introducing a bias error. Such environmental factors include variables such as temperature, vibration (acoustical and mechanical) and magnetic fields. Such factors are both time-varying and unevenly distributed throughout the coil. These environmental factors induce variations in the optical light path that each counterpropagating wave encounters as it travels through the coil. The phase shifts induced upon the two waves are unequal, producing a net undesirable phase shift which is indistinguishable from the rotation-induced signal.
One approach to attain a reduction of sensitivities arising from environmental factors has involved the use of various symmetric coil winding configurations. In such coils, the windings are arranged so that the geometrical center of the coil is located at the innermost layer while the two ends of the coil are located at the outermost layers.
N. Frigo has proposed the use of particular winding patterns to compensate for non-reciprocities in "Compensation of Linear Sources of Non-Reciprocity in Sagnac Interferometers". Fiber Optics and Laser Sensors I, Proc. SPIE Vol. 412 p. 268 (1993). Furthermore, U.S. Pat. No. 4,793,708 of Bednarz entitled "Fiber Optic Sensing Coil" teaches a symmetric fiber optic sensing coil formed by dualpole or quadrupole winding. The coils described in that patent exhibit enhanced performance over the conventional helix-type winding.
U.S. Pat. No. 4,856,900 of Ivancevic entitled "Quadrupole-Wound Fiber Optic Sensing Coil and Method of Manufacture Thereof" teaches an improved quadrupole-wound coil in which fiber pinching and microbends due to the presence of pop-up fiber segments adjacent the end flanges are overcome by replacing such pop-up segments with concentrically-wound walls of turns for climbing between connecting layers. Both of the aforementioned U.S. patents are the property of the assignee herein.
While appropriate coil winding techniques minimize some of the bias errors found in the output of a fiber optic gyro, they are not capable of eliminating all of such errors. In particular, the design of the gyro sensor coil can impact the gyro's random walk, bias stability, bias temperature sensitivity, bias temperature-ramp sensitivity, bias vibration sensitivity, bias magnetic sensitivity, scale factor temperature sensitivity, scale factor linearity and input axis temperature sensitivity.